Origin and fate of the secondary nitrite maximum in the Arabian Sea

Abstract

The Arabian Sea harbours one of the three major oxygen minimum zones (OMZs) in the world's oceans, and it alone is estimated to account for ~10–20 % of global oceanic nitrogen (N) loss. While actual rate measurements have been few, the consistently high accumulation of nitrite (NO₂^&minus;) coinciding with suboxic conditions in the central-northeastern part of the Arabian Sea has led to the general belief that this is the region where active N-loss takes place. Most subsequent field studies on N-loss have thus been drawn almost exclusively to the central-NE. However, a recent study measured only low to undetectable N-loss activities in this region, compared to orders of magnitude higher rates measured towards the Omani Shelf where little NO₂^&minus; accumulated (Jensen et al., 2011). In this paper, we further explore this discrepancy by comparing the NO₂^&minus;-producing and consuming processes, and examining the relationship between the overall NO₂^&minus; balance and active N-loss in the Arabian Sea. Based on a combination of ¹⁵N-incubation experiments, functional gene expression analyses, nutrient profiling and flux modeling, our results showed that NO₂^&minus; accumulated in the central-NE Arabian Sea due to a net production via primarily active nitrate (NO₃^&minus;) reduction and to a certain extent ammonia oxidation. Meanwhile, NO₂^&minus; consumption via anammox, denitrification and dissimilatory nitrate/nitrite reduction to ammonium (NH₄⁺) were hardly detectable in this region, though some loss to NO₂^&minus; oxidation was predicted from modeled NO₃^&minus; changes. No significant correlation was found between NO₂^&minus; and N-loss rates (<i>p</i>>0.05). This discrepancy between NO₂^&minus; accumulation and lack of active N-loss in the central-NE Arabian Sea is best explained by the deficiency of labile organic matter that is directly needed for further NO₂^&minus; reduction to N₂O, N₂ and NH₄⁺, and indirectly for the remineralized NH₄⁺ required by anammox. Altogether, our data do not support the long-held view that NO₂^&minus; accumulation is a direct activity indicator of N-loss in the Arabian Sea or other OMZs. Instead, NO₂^&minus; accumulation more likely corresponds to long-term integrated N-loss that has passed the prime of high and/or consistent in situ activities.